1. Field of the Invention
When displaying characters on an apparatus with a display device such as LCD (Liquid Crystal Display), characters can be displayed with high quality and at any size from a piece of data by employing an outline font.
Conventionally, the outline font is mainly of use in a computer with a high computing capacity as it requires calculation of outlines and calculation for filling inside the outline; however, the outline font has recently been used in embedded devices such as the cellular phone owing to the improvement of processing capacity.
Nonetheless, the size of stored data needs to be small so that the memory deployed in an embedded device which has a small capacity in many cases, may implement as many functions as possible with this small amount of memory.
The object of the present invention is to reduce the size of the conventional outline font data in view of the fact that the display device incorporated in an embedded device has an upper limit on the required character size.
The outline font data generated by the present invention can be incorporated into various character display devices, and therefore it has a wide range of usage. The present invention in particular has a high compression ratio regarding the font of a curve-laden typeface, which has been difficult to apply compression to for it has large data size, and therefore, the present invention has a great deal of utility value.
2. Description of the Related Art
Conventionally, there has been a method for performing an entropy compression of a coordinate value and a relative value of an adjacent coordinate using Huffman coding (see Patent Document 1), and a lossless compression method for performing compression after encoding the continuity of codes representing lines and curves lined up in the stroke order of an outline (see Patent Document 2) as a method for reducing the size of the outline font.
On the other hand, as a size reduction method using approximation, like the present invention, there is, for example, a method known for reducing a number of required bits representing each coordinate by representing an outline data design of 10000×10000 pixels with its resolution reduced to a level (for example 256×256 pixels), which does not cause deterioration of the design, resulting in a reduced data size, and for assigning an appropriate equation of a line/curve of the reduced size (see Patent Document 3). In this method, a line from the coordinate (3000, 5000) to the coordinate (3025, 4990) at 10,000×10,000 pixels is a line with a length of 0 from the coordinate (77, 128) to the coordinate (77, 128) at 256×256 pixels resolution. Therefore the line can be omitted, and can be reduced in data size.
The above lossless compression method has a problem that large-sized original data still has a large size after compression for it has a limited compression ratio.
As another method for reducing resolution, if a display device is capable of displaying 256 gradients, deviation from the original design can be compensated for by an anti-aliasing technique. However, on the actual device, even in a case that a character of 40×40 pixels or more does not need to be drawn (for example, a screen display in a part of an embedded device) when the resolution of the outline data is reduced to 40×40 pixels, the anti-aliasing technique is not sufficient for compensating for the deviation from the original design, resulting in failure to achieve the intended display. Therefore, like the above lossless compression method, there is a limit to the compression ratio.
FIG. 1 is a diagram showing a conventionally known data description method of a general outline font. In general, outline font data comprises a header describing data properties etc. (not shown in the drawings) and a part describing the outline of each character as shown in FIG. 1. In FIG. 1, as data describing a character outline, coordinates and types of lines connecting each coordinate are described in order starting from one point on a closed curve.
Patent Document 1: Laid-open Japanese Patent Application No. H06-149215
Patent Document 2: Laid-open Japanese Patent Application No. 2001-044850
Patent Document 3: Laid-open Japanese Patent Application No. 2004-516496